Unassailable

1. adjective

   immune to attack; incapable of being tampered with

   “an impregnable fortress”

   Similar: impregnable, inviolable, secure, strong, unattackable

2. adjective

   without flaws or loopholes

   “an ironclad contract”

   Similar: unshakable, watertight, bulletproof

3. adjective

   impossible to assail

   Similar: untouchable